Record-breaking gamma-ray emissions from a millisecond pulsar

Breaking old records for some statistic or another generally makes for catchy headlines, at least. If nothing else, the fact that the statistic is being tracked (whether or not by Guinness) suggests it’s a matter of more than passing interest. Now astrophysicists have come up with a new find that breaks three records at the same time. This one is a pulsar, designated J1823−3021A, located in the globular cluster NGC 6624. It’s a special subtype of pulsars – millisecond pulsars, which rotate from 100 to 1000 times per second (rotational period from 1 to 10 milliseconds).

Research just published about J1823−3021A estimates that, among millisecond pulsars, it has the strongest radiation in the gamma-ray part of the electromagnetic spectrum (about 8×1034 ergs/second), has the highest magnetic field strength, and is probably the youngest known.

Pulsars are rapidly rotating neutron stars that radiate most of their energy in narrow jets. The jets are aligned with the poles of the object’s very strong magnetic field. As with the Earth, this magnetic axis need not be parallel to the object’s rotation axis, so that we observe the strongest emissions in sharp peaks, like the beams of a lighthouse.

Neutron stars are the remnants left of core collapse supernovae whose progenitor stars were not massive enough to have left only a black hole after collapse – generally stars of less than 10 solar masses. Typically a neutron star starts life with a rapid rate of spin, which decreases gradually with time as the kinetic energy of rotation is lost through electromagnetic radiation. However, if the neutron star has a binary companion it may accrete matter from the companion, thus gaining angular momentum and increasing its rate of spin up to a rate of more than 100 rps.

Neutron stars generally have very strong magnetic fields. The best-known neutron star (which is also a pulsar) is the supernova remnant at the center of the Crab Nebula. This pulsar rotates at 30 rps, so it’s not a millisecond pulsar. The strength of its magnetic field at its surface is 3.8×1012 Gauss (G, the standard unit of magnetic strength). Earth’s surface magnetic field is about .5 G – 13 orders of magnitude less than that of the Crab pulsar. (This pulsar was recently in the news on account of occasional very strong gamma-ray flares.)

The new research estimates the magnetic field of J1823−3021A at 4.3×109 G. That’s quite a bit less than the magnetic field of the Crab pulsar, but larger than that of other known millisecond pulsars, which generally have fields in the range of 107 to 109 G.

NGC 6624 is about 27,000 light-years from Earth. It contains about 100,000 stars and possibly dozens of pulsars. J1823−3021A was detected by the orbiting Fermi Gamma-ray Space Telescope. This instrument does not have the spatial resolution to locate a particular gamma-ray pulsar within a globular cluster at the known distance. However, pulsars also emit energy at radio frequencies and therefore can be located very precisely. So a particular rotational frequency can be associated with individual pulsars. J1823−3021A has a period of 5.44 milliseconds (184 rps). In NGC 6624 most of the gamma-ray energy is pulsed with the same period, indicating that J1823−3021A far outshines all the other pulsars in the cluster at gamma-ray frequencies.

From observable quantities (period, luminosity, etc.), the approximate age of the pulsar and the strength of its magnetic field can be estimated – assuming a particular model of the life-history of a pulsar. J1823−3021A is now actually spinning faster than its original rate. This would happen if it has accreted mass from a binary partner, thus adding to its angular momentum. After some period of time this accretion will slow down, and so will the rotation rate, as the kinetic energy of rotation is gradually drained away in the electromagnetic radiation. No pulsars have been found with a period of much more than about 8 seconds.

The age of J1823−3021A is estimated from the rate at which its rotational period is increasing, using other observable characteristics. Since the pulsar is radiating so strongly, its spin rate must be decreasing rapidly, and so it can’t be very old. The age estimate comes out to be less than 25 million years, which is likely to be less than that of any other known millisecond pulsar. Most millisecond pulsars are much older, around a billion years of age. (Other pulsars are known that are much younger, such as the Crab pulsar, which is the remnant of a supernova that was observed in 1054 CE, just over 1000 years ago.)